As a filament for toothbrushes, there have been proposed a filament which is made of a single synthetic resin material and whose cross section perpendicular to the lengthwise direction is formed into a rectangular shape so that the contact area of surface contact between the filament and the surface of teeth is set to be large, thereby improving the stain removal property (See Patent Document 1, for example), and a core/sheath composite filament which is composed of a bristle which is formed from an ultrafine fiber made of polyamide and a rubber layer which covers the bristle, and has less irritation to gums (See Patent Document 2, for example).
Further, as a core/sheath composite filament whose cross section has a shape other than a circular shape, there have been proposed a core/sheath composite filament having a core and a sheath which are respectively made of polyethylene terephthalate and polybutylene terephthalate (hereinafter, abbreviated as PBT), polyester elastomer and PBT, polypropylene terephthalate and PBT, polyethylene naphthalate and PBT, polyester elastomer and polyethylene terephthalate, or the like, wherein a cross section of the core, the cross section being perpendicular to the lengthwise direction of the fiber, is formed into a star shape, an eight-leaf shape, or the like, and a cross section of the sheath, the cross section being perpendicular to the lengthwise direction of the fiber, is formed into a triangular shape, a square shape, or an eight-leaf shape (See Patent Document 3, for example), and a core/sheath composite filament which includes a conductive polymer which is made of polyester containing conductive particles as a sheath component and a non-conductive polymer which is made of polyethylene terephthalate as a core component, wherein a cross section of the core, the cross section being perpendicular to the lengthwise direction of the fiber, is formed into a circular shape or a triangular shape, and a cross section of the sheath, the cross section being perpendicular to the lengthwise direction of the fiber, is formed into a triangular shape or a quadrangular shape (See Patent Document 4, for example).